: Because oncogenic DNA viruses establish life-long persistent infections in humans, continuous immunosurveillance for virus-transformed cells is required to prevent neoplasia. Antigen-specific CD8+ T lymphocytes are critical in vivo effectors for eliminating virus-infected and virus-transformed cells. Investigation into the induction, expansion, and regulation of CD8+ T cells specific for these viruses is hindered by the lack of tractable animal models that mimic natural infection. Polyoma virus persistently infects the mouse, its natural host. When inoculated into immunocompromised mice or immunocompetent mice belonging to particular inbred H-2k strains, this murine Papovavirus induces a broad array of epithelial and mesenchymal cell-derived tumors. Resistance to polyoma virus tumorigenesis is mediated by polyoma-specific CD8+ T cells. In tumor-resistant H-2 mice, the polyoma virus-specific CD8+ T cell response is predominantly directed against a nine amino acid epitope from the viral Middle T oncoprotein. In vivo visualization of these dominant anti-polyoma CD8+ T cells using a class I MHC tetramer and direct ex vivo assays for their cytotoxic and cytokine effector activities reveal that mice susceptible to polyoma virus tumorigenesis mount a small, functionally impaired anti-polyoma CD8+ T cell response to this epitope. The overall goal of this applicationis to elucidate the mechanism(s) responsible for the failure of polyoma virus-specific CD8+ T cells in these mice to protect against polyoma-induced tumors. Studies are proposed to determine whether resistant and susceptible H-2k mice differ in (1) the TCR repertoire and affinity of polyoma virus-specific CD8+ T cells, (2) the magnitude and hierarchy of CD8+ T cells directed to subdominant polyoma epitopes, and (3) the antigenic specificity and cytokine profile of their polyoma-specific CD4+ T cell responses. Both physical (i.e., class I MHC tetramers) and functional (i.e., intracellular cytokine, cytokine ELISpot, and cytotoxicity) assays will be used to monitor polyoma-specific T cell responses in vivo. Elucidation of the requirements for effective CD8+ T cell surveillance against polyoma tumors will provide a valuable framework for designing strategies to bolster immunity to virus-induced neoplasia.